1. Technical Field
The present invention relates to a digital carrier multiplexer having a cross connect capability and, more particularly, a bus interface module which operates to permit cross connection of digital carrier and, in particular, pulse code modulated (PCM) channels at the DS0 T carrier hierarchical level of digital transmission.
2. Description of the Prior Art
The standard for digital carrier multiplexers operating to multiplex digital DS1, DS1C and DS2 T carrier transmission lines into a DS3 transmission line is set forth and discussed in the Bell System Transmission Engineering Technical Reference entitled "Digital Multiplexes, Requirements and Objectives" by the Director, Exchange Systems Design, AT&T (July, 1982). Digital multiplexers which are connected into the Bell System pulse code modulated T carrier telecommunications network must conform with this standard.
As is well known, a single PCM channel, known as a "DSO" channel, operates at 64 kilobits per second (Kb/sec) in each direction of transmission to transmit and receive 8,000 8-bit samples per second of a desired telecommunication, be it voice or data. According to the Bell standard, individual two way channels are multiplexed into higher speed channels for long distance transmission. As a particular example, 24 DSO channels may be multiplexed into a "DS1" channel or hierarchical level operating at 1.544 Mb/sec. In this format, 24 8-bit samples, one from each DSO channel, are arranged serially in a single transmission frame together with a single framing bit to form a 193-bit frame. Transmission of successive 193-bit frames at a rate of 8,000 frames per second determines the bit rate of 1.544 Mb/sec. Set forth in the following table are the Bell standard digital transmission lines or hierarchical levels with their associated transmission rates and numbers of channels:
TABLE ______________________________________ Transmission Number of Voice Line Channels Transmission Rate ______________________________________ DS0 1 64 Kb/sec. DS1 24 Approx. 1.5 Mb/sec. DS1C 48 Approx. 3 Mb/sec. DS2 96 Approx. 6 Mb/sec. DS3 672 Approx. 45 Mb/sec. ______________________________________
A so-called "M13" multiplexer, for example, is known which multiplexes 28 DS1 transmission lines into a single DS3 transmission line. This is accomplished by providing seven M12 multiplexers, each of which multiplexes four DS1 lines into a single DS2 line, and providing a single M23 multiplexer which multiplexes seven DS2 lines into a single DS3 line.
Additional flexibility is provided by a so-called "MX3" multiplexer. The MX3 multiplexer is capable of connecting different numbers of DS1, DS1C, and DS2 lines to a single DS3 line. For example, seven DS2 lines connected to the M23 multiplexer are formed by concentrating one group of four DS1 lines, concentrating one group of two DS1C lines or by direct connection of a DS2 line. As will be appreciated, the "X" in the MX3 multiplexer designation refers to the fact that three different types of transmission types of transmission lines--namely DS1, DS1C, and DS2--are connectable to this device.
A more recent known development is a complete switching "node" which connects a variable number of DS1, and DS1C, lines as inputs and outputs and routes any voice channel on any one line to a voice channel on another line. This is accomplished with the aid of a digital cross-connect system (DCS) which operates in the manner of a telephone exchange to connect any incoming transmission line to any outgoing transmission line. One such digital cross-connect system is currently marketed by AT&T Technologies, Inc. under the acronym "DACS" (Digital Access and Cross-Connect System). This known cross-connect system provides a crosspoint array to enable any DS1 or DS1C transmission line of one transmission rate to be connected to another DS1 or DS1C transmission line. The system is unable to perform multiplexing at the DS2 or DS3 level.
However, separate conversion from one transmission rate to another may be effected by a plurality of separate MX3 multiplexers. Furthermore, M12 and MC2 multiplexers may also be provided, as desired, to convert to a DS2 transmission rate.
While multiplexing equipment of this type may be tailored to voice transmission network nodes of a particular configuration, such multiplexing equipment is not readily programmable so that changes in the node configuration are expensive and time consuming. This known equipment includes dedicated multiplexer hardware which must be physically interconnected into or removed from the system each time a change is made.
In co-pending U.S. patent application, Ser. No. 683,901, filed Dec. 20, 1984 by the present inventors, there is disclosed a "Digital Multiplexer for PCM Voice Channels Having a Cross-Connect Capability". Specifically, the cross-connect capability comprises a bus system providing parallel data transmission among low and high speed modules, the low and high speed transmission data being converted from a serial to parallel format for bus transmission while each module is provided with means to provide an appropriate hierarchical serial output.